The majority of women diagnosed with ovarian cancer ultimately succumb to the disease due to the outgrowth of cells resistant to chemotherapy [4]. An improved understanding of cellular mechanisms of chemoresistance is essential in developing new effective anticancer therapy to overcome the problem of chemoresistance. We have shown that HtrA1, a serine protease with previously unknown function, is down-regulated in a majority of ovarian cancer cell lines and primary tumors. Additional studies indicated that HtrA1 may modulate cell death and anchorage independent growth of ovarian cancer cells [2]. Our more recent data indicate that HtrA1 is upregulated by cisplatin and paclitaxel and suggests that it may contribute to sensitivity to chemotherapy. Increased cell death and chemosensitivity following HtrA1 expression depends on the protease activity of HtrA1, suggesting a role of serine protease activity in programmed cell death (PCD). Moreover, 90% of patients (27/30) with tumors expressing high levels of HtrA1 responded to chemotherapy with complete or partial clinical remissions, compared to 62% (8/13) and 65% (11/17) response rates in tumors with low and moderate levels of HtrA1, respectively. Response to chemotherapy was significantly different between low and high (P = 0.0276) or moderate and high (P = 0.0342) staining groups. These findings identify HtrA1 as a novel modulator of cisplatin-induced cytotoxicity and suggest that loss of HtrA1 in ovarian cancer may contribute to chemoresistance [5]. The objective of this application is to determine the mechanisms by which HtrA1 expression is regulated in cancer and define the role of HtrA1 in programmed cell death and chemoresistance. Based on our preliminary studies, we are proposing the following three hypotheses: first, HtrA1 expression is epigenetically regulated in cancer and is transcriptionally upregulated by chemotherapy in cancer cells when its expression is not silenced by epigenetic mechanisms; second, HtrA1 participates in a serine proteasome-mediated PCD that cross-talks with caspase-mediated cell death pathways; and third, targeted degradation of specific substrates by HtrA1 following chemotherapy treatment contributes to chemotherapy-induced cytotoxicity. PUBLIC HEALTH RELEVANCE: Emerging evidence suggests that some of the same changes that contribute to neoplastic transformation also contribute to drug resistance. In particular, the same anti-apoptotic changes that contribute to the transformed phenotype by making cells resistant to the stresses of unfavorable growth conditions and loss of important homeostatic processes also appear to make cells more resistant than they would otherwise be to cancer chemotherapy. Based on this view, studies that improve our understanding of the process of tumorigenesis have the potential to also provide new insight into the problem of drug resistance. Our studies have shown that altered expression of the serine protease HtrA1 modulates chemotherapy induced cytotoxicity. At the conclusion of the proposed studies, we hope to elucidate the significance of HtrA1 down-regulation in ovarian cancer and how this reduced HtrA1 expression contributes to the development of chemoresistant ovarian cancer. A better understanding of the role of HtrA1 in a poorly understood serine protease mediated programmed cell death may contribute to discoveries of new therapeutic approaches to overcome drug resistance. Drug resistant ovarian cancer is a lethal disease, and this project will better define novel mechanisms of drug resistance and programmed cell death in ovarian cancer.